A.2 MQTT implementations

Like HTTP, the MQTT protocol is asymmetric in that it distinguishes between two different roles: client and server.

In MQTT terms, a Client is a program or device that uses MQTT. It always establishes the Network Connection to the Server. A Client can:

• Publish application messages that other Clients might be interested in.
• Subscribe to request application messages that it is interested in receiving.
• Unsubscribe to remove a request for application messages.
• Disconnect from the Server.

An MQTT Server is an entity that accepts connections from Clients. Unlike HTTP it generally does not run any application logic, instead an MQTT Server acts as an intermediary between Clients publishing application messages and the Clients which have subscribed to receive them.

The MQTT specification [1] recommends the use of IANA registered ports 1883 (MQTT over raw TCP/IP) and 8883 (MQTT running over TLS).

Although the MQTT protocol is relatively simple to implement, applications normally make use of pre-built implementations:

• The applications themselves link to libraries that provide the MQTT client functionality. Libraries are available for a variety of programming languages and operating environments.
• The MQTT server functionality can be provided by a standalone software process (possibly running on a server that is remote from the clients), a hardware appliance or a cloud-hosted MQTT service.

The Eclipse foundation through their M2M working group, provides open source MQTT client code via its Paho Project, and an open source server implementation via its Mosquitto project. Other open source and commercial implementations are also available.